Presented to the AAS at the January 2008 meeting in Austin, Texas. [AAS link]
Molecular clouds are enmeshed in webs of diffuse atomic gas that contain important information about their formation and interaction with their environment, not least because the atomic gas can trace lower column densities that are unshielded from UV radiation. The nearby molecular cloud complex MBM 53-55 (Magnani et al. 1985; Yamamoto et al. 2003) is thought to be part of an expanding shell from either stellar winds or a past supernova (Gir et al. 1994). Although this complex subtends some 15 degrees on the sky, its atomic gas has not previously been imaged at sub-parsec scales. The GALFA H I sky survey of Galactic 21cm-line emission with the Arecibo L-band Feed Array has recently mapped the MBM 53-55 region with a 3.5-arcminute beam and 0.2 km/s channels. GALFA's high resolution and sensitivity allow the H I content, environment, and kinematics of these clouds to be explored as never before. The observed H I structure matches IRAS dust emission so well that it is hard to identify IRAS filaments without H I counterparts: the GALFA survey essentially adds a velocity axis to IRAS maps. Most high column density H I features also appear to have CO emission counterparts, but there are also disagreements that may trace the gas phase evolution. We will consider the implications of the GALFA H I data for the origins of the shell-like structure and the formation of the molecular clouds.
This work is supported by the National Astronomy and Ionosphere Center, operated by Cornell University under Cooperative Agreement with the National Science Foundation.